1. Field of the Invention
The present invention relates to a gimbal assembly for flying magnetic transducer heads and, more particularly, to such a gimbal assembly which allows for a significant improvement in the ratio of in-plane stiffness to stiffness in a direction normal to the gimbal plane.
2. Description of the Prior Art
Magnetic disc storage systems are widely used to provide large volumes of relatively low cost computer-accessible memory or storage. A typical disc storage device has a number of discs coated with a suitable magnetic material mounted for rotation on a common spindle and a set of transducer heads carried in pairs on elongated supports for insertion between adjacent discs, the heads of each pair facing in opposite directions to engage opposite faces of adjacent discs. The support structure is coupled to a positioner motor, the positioner motor typically including a coil mounted within a magnetic field for linear movement and oriented relative to the discs to move the heads radially over the disc surfaces to thereby enable the heads to be positioned over any annular track on the surfaces. In normal operation, the positioner motor, in response to control signals from the computer, positions the transducer heads radially for recording data signals on or retrieving data signals from a preselected one of a set of concentric recording tracks on the discs.
As the density at which digital information is recorded on a magnetic recording surface is increased, the gap between the recording head and the magnetic recording surface must be decreased. The smaller the gap and the closer the magnetic head is positioned with respect to the recording surface, the more difficult it becomes to control the mechanical tolerances of the structure mounting the recording head. To overcome these mechanical difficulties, mechanical recording heads are placed in head assemblies adapted for floating on a thin film of air created by the laminar air flow due to the rotation of the recording surface. Modern magnetic disc drives incorporate rigid substrate discs, the surfaces of which are polished to a high finish so that the head can reliably fly on the air bearing. Systems are presently being developed wherein the heads fly above the disc recording surfaces at heights of less than 20 microinches.
In such systems, when the recording medium rotates, the laminar air flow causes the head assembly to be forced away from the medium. Therefore, some urging means, such as a spring, must be provided to overcome this air flow and counterbalance the head assembly, keeping it as close to the recording medium as possible. Furthermore, floating magnetic recording head assemblies are often mounted in gimbal mounting devices in order to allow the angle and position of the magnetic recording head assembly to conform to the air bearing.
Even with spring loaded gimbal assemblies, disc and drum dimensions and spindle bearing run-out can not be controlled accurately enough to prevent small but significant variations in the clearance between the disc and the head during operation. Variations may be very slight, but when dealing, as here, with head-to-recording surface clearances of less than 20 microinches, even minor deviations can be disastrous because the air film supporting the head assembly adjacent the surface can then be penetrated by the head. When this occurs, the recording surface and head assembly abrade each other and the particles so formed cause even more abrasion to occur in an avalanche effect which soon destroys both recording surface and head assembly. Accordingly, it is necessary to permit the head to move perpendicularly to and rotate about pitch and roll axes parallel to the recording surface to maintain the desired clearance between the two. It should be understood that this movement, while only a matter of microinches, is extremely rapid because of the high disc speeds involved. It is necessary that while the head is shifting to accommodate these changes, it should not be moved from registration with the track beneath it.
In order for the head to follow rapid movement of the recording surface, it is necessary, first of all, that the head and the supporting structure should be as light as possible to decrease mass which must be accelerated by the forces of the air film. Secondly, the head must be suspended very rigidly with respect to its support, so as to prevent translation of the head relative to the arm and parallel to the recording surface, as this will interfere with transcribing. Rotation of the head about an axis normal to the recording surface must also be prevented so as to keep the gaps parallel to the individual bit patterns at all times.
To summarize, the head suspension can be considered as a six-degree-of-freedom system. These six degrees are rotation and translation about two orthogonal axes (roll and pitch) parallel to the recording surface and the axis normal thereto. The ideal mount should have a very low spring rate for rotation of the head about any axis parallel to the recording surface. The spring rate for translation along an axis normal to the recording surface must be controlled quite closely to maintain the proper head-to-surface clearance. The assembly should have very little sprung weight. On the other hand, the head should be mounted so as to have very high spring rates for translation of the head parallel to the recording surface and in rotation about an axis normal to it.
The most effective apparatus used heretofor for achieving the desired result employs a gimbal sheet formed from a single, thin, approximately square, piece of resilient material, such as steel, for attaching a transducing head to a head arm. Typical gimbal sheets are disclosed in U.S. Pat. No. 3,896,495 (Beecroft) and U.S. Pat. No. 4,058,843 (Gyi). In the Beecroft patent, the gimbal sheet is flat. The head arm itself is as rigid as possible to prevent any appreciable deflection of it during operation. The periphery of the gimbal sheet is attached at mounting points on its opposite edges to a side of the cantilevered head arm end so as to be positioned generally parallel to an adjacent recording surface in such a manner that a clearance space between the arm and every part (except for the mounting points) of the gimbal sheet exists. This is accomplished by mounting this sheet across an opening in the arm end. Two cut-out portions of the sheet define inner edges of an outer or peripheral ring of the sheet, which ring is the element carrying the points attached to the arm. The distance between the outer edge and the adjacent inner edge is preferably several times greater than the thickness of the sheet itself. The entire inner edge of the outer ring is not cut free, but two webs on opposite sides of the sheet remain attached to and integral with the external ring, one on each side of the sheet, between the edges attached to the head arm. These webs are free to, in effect, rotate about a pitch axis parallel to the recording surface and passing approximately through the mounting points, by bending of the outer ring.
The webs are also integral with an inner area whose outer edge is defined by the cut-out portions previously mentioned. An opening is also present within the inner area which transforms into a ring slightly smaller than the outer ring, but having a similar difference between the inner and outer dimensions. The head itself is attached across this opening at two points on the inner ring forming a clearance between itself and all other objects. When so positioned, the head is suspended from the arm by the gimbal sheet and direct contact between the head and arm is, at the most, only through a load spring. The attachment points of the head to the gimbal sheet are to the innner ring and are preferably on a line approximately parallel to the pitch axis.
From an inspection of the Beecroft patent, it can be seen that the head can rotate about a roll axis parallel to the recording surface and normal to the pitch axis with very little resistance from the gimbal sheet by bending the relatively long cantilevered sides of which the outer and inner rings are comprised. Their relative thinness in the direction normal to the recording surface allows elastic deflection of them quite easily. For this reason, the head can also be easily translated along an axis normal to the recording surface. However, resistance to rotation around this axis is very high since, in this case, either compression or tension loads are placed on the ring sides or bending moments about axes normal to the recording surface are placed on them. For the same reasons, the gimbal sheet strongly resists translation along the roll and pitch axes.
If the gimbal sheet is made laterally symmetrical with respect to both the pitch and roll axes, translation along the axis normal to the recording surface will be perfectly straight line and cause no rotation of the head about this axis. Thus, the head can be expected to precisely follow the recording track beneath it regardless of variations in its height. Furthermore, only a very small amount of mass in addition to the head is moved whenever the head position is changed. Thus, forces necessary to accelerate the head at a given rate are much smaller than those previously required. This allows the transducing head to be flown closer to the recording surface, a desirable condition, with fear that the head will strike the recording surface due to small changes in its height. The force generated by the air bearing as the height changes is sufficient to accelerate the head away from the recording surface quickly enough to avoid their touching.
Secondly, since the roll and pitch axes are quite close to the air bearing surface of the head, as roll and pitch occurs, translation of the head along axes parallel to the recording surface is minimized. This is particularly important when the head rolls, because this causes translation of the head normal to the data track and parallel to the recording surface. This lack of registration between the head and track must, when the entire arm moves, be compensated for by repositioning of the carriage carrying the arm and the net result is less precision in head position.
When making a gimbal sheet, as in Beecroft, Gyi, and other similar systems, it is desirable to make the gimbal sheet as thin as possible to provide for as low a spring rate as possible for translation along an axis normal to the recording surface. As a practical matter, a limit is reached as to the degree of thinness possible because of fabrication and handling problems. Thus, there has been an upper limit to the ratio of stiffness of the gimbal sheet in the plane thereof to stiffness normal to the plane thereof. Significant improvements in this ratio have been unobtainable heretofor.